1. Field of the Invention
The present invention relates to a semiconductor laser control method and a semiconductor laser control device preferably used for a reading control system such as a bar code reading device.
2. Description of the Related Art
In recent years, bar codes have been extensively utilized typically in POS (point of sales) system in the distribution industry. Such a circumstance demands a small, low price, low power consumption bar code reading device. In order to answer such needs, semiconductor lasers have been used in place of conventional gas lasers.
FIG. 23 is a structural block diagram showing a bar code reading device in which a semiconductor laser control device is used in the optical system. In FIG. 23, numeral 1 represents a bar code printed on a surface of an article. The bar code 1 is formed of black bars and white bars arranged alternately and indicates specific datum based on the widths thereof.
The optical system 2 irradiates a laser beam L2 to the bar-code and receives the laser beam R1 reflected from the bar-code 1. The optical system 2 also is formed of a semiconductor laser control device (laser emitter) 3, a scanning mechanism 4, and a photoelectric converter 5.
When the semiconductor laser control device 3 is activated under the control of a CPU, it amplifies the laser output based on a predetermined time constant and holds automatically its light amount at a predetermined value.
As shown in FIG. 24, the semiconductor laser control device 3 is constituted of a laser diode chip (LD chip) 30, a transistor 31, an analog switch (SW) 34, operational amplifiers 36 and 38, a reference voltage source 42, a capacitor 35, a variable resistor 37, and resistors 32, 33, and 46. The LC chip 30 is formed of a laser diode (LD) 11 as a semiconductor laser, and a photo diode 12.
In such a circuit configuration, an external photo diode 12 detects a current Im showing a laser light amount. As shown in FIG. 25, the operational amplifier 36 operates so as to equalize the voltage Va with a reference voltage Vref so that the LD drive current Iop from the semiconductor laser is subjected to a feedback control to maintain the laser light amount at a constant value.
In the operation, as shown by numerals (1) and (2) in FIGS. 25, when the analog switch 34 is turned off (see time a) in response to a control signal from the CPU, the variable resistor 37 converts the monitoring current Im as the detected laser light amount into a voltage Va corresponding to a predetermined light amount.
The voltage VA (or VA1) is compared with the reference voltage Vref and is integrated by the operational amplifier 36 so that the output voltage VB of the operational amplifier 36, as shown in FIG. 25, rises, for example, in accordance with a time constant of TcmV/sec from the reference voltage Vref. Then the output voltage VB of the amplifier 36 is converted into an LD drive current Iop by the transistor 31.
When the voltage Va exceeds a threshold voltage after a period b of time, or the LD drive current Iop exceeds a threshold current value of the laser diode 11, the laser diode 11 starts its laser oscillation.
Then the current Im from the external photo diode 12 increases proportionally to the light amount while the voltage VA inputted to the negative input of the operational amplifier 38 increases during a period from the time b to the time c as shown in FIG. 25.
The output of the operational amplifier 36 is increased till the voltage VA is equalized with the reference voltage Vref. The operational amplifier 36 equalizes the voltage VB with the reference voltage Vref so as to maintain the output voltage VB at a fixed value.
As a result, since the signal current sent to the base of the transistor 31 is at a fixed value, the LD drive current Iop is maintained at a fixed value, thus maintaining a laser output of the laser diode 11 at a predetermined value. The voltage changing rate depends on the reference voltage Vref, the capacitor 35, and the resistor 46.
When the voltage reaches a predetermined value, the operational amplifier 36 compensates variations in laser output variations due to various factors with the time constant so as to maintain the predetermined value. When the analog switch 34 is turned on in response to the control signal from the CPU, the operational amplifier 36 provides the output at the reference voltage Vref so that the transistor 31 is turned off to stop the laser oscillation.
The LD drive current Iop of the laser diode 11 is increased in accordance with the time constant because an abrupt LD drive current Iop producing a predetermined light amount (after the time c in FIG. 25) may destroy the laser diode 11.
FIG. 26 shows a modified semiconductor laser control device 3A. In the semiconductor laser control device 3A, resistors 62 and 63 are connected to the negative input of the forgoing semiconductor laser control device. A resistor 61 is added instead of the variable resistor 37 and a variable resistor 25 is inserted between the resistor 46 and the operational amplifier 38.
The arrangement of the resistors 61, 62, and 63 converts the current Im inputted to the positive and negative inputs of the operational amplifier 38 into a voltage with a predetermined ratio. The arrangement allows the variable resistor 25 to arrange to the output of the operational amplifier 38 and to divide the voltage at the output of the operational amplifier 38. Other configuration is similar to those in the semiconductor laser control device 3.
In the semiconductor laser control devices 3 and 3A, the reason that the variable resistors 37 and 25 are connected to the positive terminal of the operational amplifier 38 and the input of the resistor 46 will be explain ed below.
As shown in FIG. 27, even if the laser diode 11 and the external diode 12 used in the LD chip 30 are made of the same kind, the mounting condition may cause variations several times in the monitoring current Im of the laser diode 11 under the same light amount. For that reason, a proper negative input of the operational amplifier 36 may not be established with respect to light amount. However, the voltage division due to the above layout of the variable resistor allows the voltage sent to the operational amplifier 36 to adjust at a proper value.
FIG. 27 shows the relationship between the voltage division ratio K and the current Im at the optical amount P5 of 5 mW. The scale of the voltage division ratio K is proportional to the angle of the resistance varying knob of a variable resistor. As shown by the light amount over the range between 2 mW to 4 mW in FIG. 27, the more the current Im increases, the more the range between 2 mW and 4 mW narrows. There are the relations of the current Im=(light amount P0).times.(light amount P5)/5 mW, the voltage Vm=(current Im).times.(resistance value of the resistor 61), and K=(reference voltage Vref).div.(voltage Vm), where light amount P0 is a light amount value varied with a voltage division ratio K with respect to a specific current Im.
In some cases, as shown in FIG. 28, a metal heat sink 28 is directly and electrically contacted to the LD chip 30 to dissipate the heat generated from it.
The scanning mechanism 4, shown in FIG. 23, is formed of a polygon mirror driven rotatably by, for example, a motor, and reflects the laser beam L1 from the laser emitting unit 3. The laser beam L1 irradiates as a laser beam L2 to the bar code 1 including black bars and white bars and moves and scans at a fixed rate and perpendicularly to the bar code 1.
The scanning mechanism 4 irradiates the reflected light R1 as the reflected light R2 to the photoelectric converting unit 5 while the reflected light R1 moves with the scanning of the laser beam L2.
Furthermore, the photoelectric converting unit 5 is formed of an photoelectric element such as a photo diode, and converts the reflected light (optical input signal) R2 received via the scanning mechanism 4 into an electric signal (analog value) corresponding to the light amount.
Numeral 6 represents an a/d converter for converting an electric signal from the photoelectric converting unit 5 to a digital signal. The aid converter 6 also converts a binary signal including a black level signal corresponding to the black bar portion of the bar code 1 and a white level signal corresponding to the white bar portion thereof by converting an electric signal from the photoelectric converting unit 5 in a digital form. In the binary signal, since the light amount of the light reflected from the white bar portion is larger than that from the black bar portion, the white level signal is obtained as a high level signal and the black level signal is obtained as a low level signal.
Numeral 7 represents a bar width counter for counting the clock signal from the clock generator 8. The bar width counter 7 outputs the time widths of the black level signal portion and the white level signal portion from the a/d counter 6 or values corresponding to the black bar width and the white bar width of an actual bar code 1, as a counted value of the clock signal.
Numeral 9 represents a memory for storing a bar counted value from the bar width counter 7 and 10 represents a CPU. The CPU 10 extracts and demodulates specific data of the bar code 1 based on the bar width counted values (values corresponding to the each black bar width and each white bar width) stored in the memory 9.
In the above structure, the laser beam L1 emitted from the laser emitting unit 3 is irradiated as the laser beam L2 to the black bar and the white bar of the bar code 1 by the scanning mechanism 4, and is moved and scanned at a fixed rate and perpendicularly to the black bar and the white bar thereof.
The laser beam L2 from the scanning mechanism 4 is scattered and reflected by the bar code 1 and reentered as a light R1 reflected back to the scanning mechanism 4. The reflected light R1 moves at the scanning rate of the laser beam L2 while the reflection angle varies. When the reflected light R1 is reflected on the polygon mirror forming the scanning mechanism 4, it enters onto the photoelectric element of the photoelectric converting unit 5 arranged at a predetermined place as the reflection light R2.
The photoelectric converting unit 5 converts the reflected light R2 into an electric signal corresponding to the light amount thereof. The a/d converter 6 converts the electric signal to a signal in a digital form or a binary signal having a black level signal corresponding to a black bar portion in a bar code 1 and a white level signal corresponding to a white bar portion therein.
Then the bar width counter 7 counts the clock signals from the clock generator 8 and measures the time widths (values corresponding to the widths of a black bar and a white bar of an actual bar code 1) of the black level signal portion and the white level signal portion of a binary signal from the aid converter 6, and stores the counted value of the clock signals temporarily into the memory 9. The CPU 10 subjects the bar width count value stored in the memory 9 to a specific demodulation process to extract and demodulate specific data from the bar code 1.
However, the semiconductor laser controller used for the above bar code reader cannot obtain the normal feedback current and voltage if noise due to a factor is induced in the feedback system when the laser diode emits properly a light beam of a predetermined light amount.
Thus the operational amplifier varies the output with a given time constant or the time constant ranging from an emitting time to the time to a predetermined light amount even if the voltage is enough to establish a predetermined light amount. However, there is a disadvantage in that since this time constant provides an excessive varying rate, the laser diode emits abnormally before the feedback system in abnormal state is detected.
Furthermore, there is a disadvantage in that the laser output is not stabilized at a predetermined light amount because the above time constant varies largely when the laser output is compensated to a predetermined light amount.
As shown in FIG. 27, when the current to a light amount is large, the scale in light amount is narrowed. Hence the voltage division ratio K between 0.82 to 0.42 can be easily adjusted by a variable resistor. However, when the ratio is less than 0.42, the light amount adjusting becomes difficult because the light amount varies coarsely at a slight variation in angle of the knob of the variable resistor. A heat sink mounted on a LD chip may conduct an undesired current through the LD chip 30 because the heat sink in a shape provides an antenna effect to an electromagnetic wave and electrostatic electricity. As a result, there is a disadvantage in that the operational life of the laser diode in the LD chip and an external photo diode becomes short.